Installation instructions issues for Release 21 [Windows]

[mikehaojiang]

Following instructions (https://github.com/Unity-Technologies/ml-agents/blob/develop/docs/Installation.md) did not work for Windows 10 due to the following issues:

• conda create -n mlagents python=3.10 && conda activate mlagents --> This one should be the following instead: conda create -n mlagents python=3.10.12 && conda activate mlagents
• python -m pip install ./ml-agents-envs causes an error for numpy==1.21.2 (cannot build wheel). I had to change numpy==1.21.3 in the ./ml-agents-envs/setup.py for it to work

[miguelalonsojr]

Fixed with #6014